1. Field of the Invention
The present invention relates to a stacked resonator with a plurality of conductors stacking one upon another, and a filter constructed by using the stacked resonator.
2. Description of the Related Art
For example, demanding requirements of miniaturization and minimum loss are placed on filters used in radio communication equipments such as cellular phones. Consequently, the same is true for resonators constituting the filters. As a filter having a balanced terminal, there is known for example a band pass filter of unbalanced input/balanced output type. As such a filter, there is one using a balun. The balun is used to perform mutual conversion between an unbalanced signal and a balanced signal. In a line for transmitting an unbalanced signal, a signal is transmitted by the potential of a signal line with respect to a ground potential. In a line for transmitting a balanced signal, a signal is transmitted by the potential difference between a pair of signal lines. A balanced signal is generally considered as being superior in balance characteristics when the phases of signals transmitted between a pair of signal lines are different from each other by 180 degrees, and are of substantially the same amplitude.
FIG. 23 illustrates a general structure of a balun. This balun has a half-wave (μ/2) resonator 201, and first and second quarter-wave resonators 202 and 203. Both ends of the half-wave resonator 201 are open ends, and an unbalanced input terminal 211 is connected to one open end. The short-circuit ends of the first and second quarter-wave resonators 202 and 203 are arranged so as to oppose to the half-wave resonator 201 so that they are opposed to the open ends of the half-wave resonator 201, respectively. Balanced output terminals 212 and 213 are connected to the open ends of the first and second quarter-wave resonators 202 and 203, respectively, thereby forming a pair of balanced output terminals.
As a balun having this structure, there are laminate type balun transformers as described in Japanese Unexamined Patent Publications No. 2002-190413 and No. 2003-007537. Both aim at miniaturization due to a laminate structure which can be obtained by forming each resonator with a spiral-like conductor line pattern, and forming the conductor line pattern on a plurality of dielectric substrates. Japanese Unexamined Patent Publication No. 2005-045447 and No. 2005-080248 describe laminate type band pass filters using a half-wave resonator, as a balanced output type band pass filter.